The present invention relates to optical markers and, more specifically to tracking optical markers in a video sequence.
With regard to the concept of beacon markers and by way of background it is noted that the optical markers are visible structures with known positions in a 3-dimensional world. It is herein recognized that by identifying and localizing such markers in video frames, one can calculate the corresponding poses of a video camera having known internal camera parameters. The camera pose contains six parameters, corresponding to three translational and three rotational degrees of freedom.
A 3-dimensional computer generated models can be designed to exhibit geometric attributes of a real object as they would appear to a xe2x80x9cvirtual observerxe2x80x9d located at a xe2x80x9cvirtual viewpointxe2x80x9d.
Knowledge of the camera pose allows one, for example, to render 3-dimensional computer generated models onto the video sequence such that the virtual viewpoint of these models is matched to the xe2x80x9crealxe2x80x9d viewpoint of the video camera. The computer models then appear from the viewpoint of the video camera as an integral part of a 3-dimensional scene.
The achievement of optical marker tracking in real time presents certain difficulties, as will be explained. Real time tracking is usually performed in two phases: first, an initialization phase, where the whole video frame is searched for the markers and the initial marker locations are determined and, second, an actual real time tracking phase wherein the marker locations are constantly updated, taking their previous locations into account. The neighborhood of an old marker location is searched to find the new marker location. At present, standard computers do not provide sufficiently fast computation to allow one to search through the full video frames in real time, typically at 25 or 30 frames per second.
The real time tracking phase is inherently non-robust. A sudden sharp movement or jolt of the camera may displace the markers in the video frame to the extent that they xe2x80x9cfallxe2x80x9d outside their search windows, whereupon the system then loses track of the markers, requiring a new initialization phase.
To improve the robustness of optical tracking systems, it has been proposed to supplement them with non-optical, e.g. magnetical trackers. See, for example: State, Andrei, Gentaro Hirota, David T. Chen, William F. Garrett, and Mark A. Livingston. xe2x80x9cSuperior Augmented-Reality Registration by Integrating Landmark Tracking and Magnetic Tracking.xe2x80x9d Proceedings of SIGGRAPH 96. (New Orleans, La., Aug. 4-9, 1996); Computer Graphics Proceedings, Annual Conference Series 1996, ACM SIGGRAPH, pgs. 429-438.
It is herein recognized that such a hybrid tracking system would use the non-optical tracker to give a robust, yet not very precise estimate of the camera pose, while the optical tracker would refine this estimate and yield the marker locations with improved precision. The robustness of the hybrid system comes at the price of increased cost and complexity of the system.
In accordance with an aspect of the invention, a method for optical tracking in a video sequence comprises the steps of placing optical markers in a 3-dimensional scene for viewing by a camera from a given camera pose. Certain preselected markers are designated optical beacons and are configured for ready location of images thereof in an image of the camera.
In accordance with an aspect of the invention, others of the markers are placed in positions having known relative spatial relationship to the optical beacons. Images of the optical beacons are found in the image. Images of a plurality of others of the optical markers are found by using existing knowledge of their relationship to the beacons, optionally via an approximate camera pose corresponding thereto that is derived from the image information, precise locations of optical markers are obtained, and information of the precise locations of optical markers is uses to obtain a precise camera pose.